Ultramagnesian Olivine in the Monchepluton (Fo96) and Pados-Tundra (Fo93) Layered Intrusions (Kola Peninsula)

Abstract

Abstract —The paper focuses on compositional variations of olivine and chromian spinel in the Monchepluton and Pados-Tundra layered intrusions, which host significant chromitite mineralization. Ore-bearing dunite (with up to 25–30 vol.% Mcr) in the Sopcheozerskoe chromite deposit from the Monchepluton complex, Kola Peninsula, Russia, bears an assemblage of phases with exceptionally high magnesium contents: Fo96 + augite (Mg# = 94) + magnesiochromite, Mcr (Mg# ≈ 65); Mg# = 100·Mg/(Mg + Fe2+ + Mn). However, olivine in the host dunite has normal maximum values of Mg# comparable to those in cumulus olivine from layered intrusions worldwide (Fo≤91–92). The Fo96 phase in the Sopcheozerskoe deposit shows the most primitive composition ever reported from any layered intrusion. Magnesiochromite occurs as unzoned homogeneous euhedral crystals unaffected by subsolidus exchange or metasomatic effects. Olivine in ore-bearing dunite (20–25 vol.% magnesian chromite) from the Pados-Tundra complex attains Fo93, with the Mg# value notably higher than the range (Fo85.5–90.6) in olivine from orthopyroxenite, harzburgite, and dunite within the intrusion. Olivine and chromian spinel in the two complexes behave coherently, with covarying patterns of Mg# and Ni contents in olivine at R = 0.75 (n = 160) and positive correlation between Mg# in coexisting chromian spinel and olivine grains at R = 0.8 (n = 150). This behavior indicates that the two phases attained equilibrium during crystallization. It appears unlikely that the extremely high Mg enrichment in olivine (Fo96), as well as in all associated phases of the Monchepluton complex, would result from a subsolidus reaction between olivine and chromian spinel or low-temperature alteration of olivine. We suggest a more realistic explanation that the olivine (+ high-Mg augite)–chromian spinel assemblage crystallized from komatiitic magma under the conditions of progressively increasing oxygen fugacity (fO2). The high Mg# in the Mcr-chromite-enriched system, above the maximum values common in cumulus olivine from layered intrusions (up to Fo96 against Fo≤91–92), may be caused by shortage of ferrous iron.